(1) Field of the Invention
The present invention relates to an optical head. More particularly, the invention relates to an optical head including as a light source a semiconductor laser having a plurality of lasing points.
(2) Description of the Prior Art
There has heretofore been adopted a method in which video or digital information is recorded and reproduced on a recording material (for example, a thin metal layer or thin calcogenite glass layer) formed on a disc or drum by using as a light source a laser beam from a smallsize compact semiconductor laser. In accordance with this method, it may be considered to use a semiconductor laser array having a plurality of lasing points as the semiconductor laser so that some of the lasing points are used as the light source for recording of information and the remaining lasing points are used for reproduction of information, for focusing error signal detection and for tracking error signal detection.
However, in such conventional semiconductor laser arrays, the respective lasing points are formed at intervals of about 100 to about 200 .mu.m, and therefore, laser beams from the lasing points 1a and 1b on the semiconductor laser array 1 are changed to laser beams 3a and 3b after passage through a coupling lens 2, as shown in FIG. 1. It can be seen that, as the distance from the coupling lens is increased, the beams 3a and 3b are separated from each other remarkably. As a result, the majority of the laser beams pass beyond the lens aperture of the laser beam focusing lens 4. For example, let it be assumed that the distance between the lasing points 1a and 1b is 200 .mu.m, the magnification of the coupling lens 2 is 20, the distance from the lens 2 to the imaging point is l.sub.1, the magnification of the focusing lens 4 is 20 and the distance from the lens 4 to the imaging point is l.sub.2 (which is equal to l.sub.1), the distance between the beams 3a and 3b on the imaging plane P is 4 mm (equal to 200 .mu.m.times.20) and this distance on the focusing lens is 8 mm. Further, assuming that the aperture diameter of the focusing lens is 6 mm, the beams 3a and 3b are hardly introduced at all into the aperture of the focusing lens. Accordingly, this problem cannot be solved without approximating the points 1a and 1b to each other very closely. However, from the viewpoint of the thermal and electronic interferences of two lasing points, the distance between the points 1a and 1b should be at least about 100 .mu.m. Even in the case, the distance between the beams 3a and 3b on the focusing lens 4 is 4 mm and therefore, the truncation of the beams becomes very large. When there are three or more lasing points, beams are not substantially introduced into the focusing lens.